1. Field of the Invention
This invention relates to an improved process of electrolessly depositing copper on an insulating substrate. This invention also relates to an improved method of removing resin smears on interior walls of holes formed in insulating substrates.
2. Description of the Prior Art
Hole forming operations in resin containing materials often result in the smearing of resin over the interior wall or barrel of a hole. This resin smear is primarily attributable to the generation or utilization of temperatures exceeding the melting point of a resinous component of the material during hole forming operations.
When holes are drilled in epoxy impregnated fiberglass laminate materials, for example, friction of the drill bit against the material raises the temperature of the bit. Numerically controlled drilling machines are in common use today, and they are capable of a very rapid drilling sequence, up to 200 holes per minute. Under these conditions, drill bit temperatures of 260.degree. C. to 315.degree. C. occur, and such temperatures exceed the melting temperature of many resin systems. The drill bit thus picks up melted resin on its course through the material being drilled, and this melted accretion is smeared in the barrel of the hole. In laser drilling to contact interior conductors in organic insulating substrates, a similar resin accretion or smear is developed on the exposed conductor surface.
While the resin smeared on the hole walls may be ignored in some applications, it is at times imperative that it be removed. Consider, for example, a hole that has been formed in a resin-containing material which includes a plurality of parallel planar metallic conductors, with the hole perpendicular to, and communicating with, two or more of the parallel metallic conductors, i.e., a multilayer circuit board. If it is desired to metallize the hole walls in order to form a conductive path between two or more of the metallic conductors, the resin smear must be removed from the edges of the hole through the metallic conductors if conductive contact between the metallized hole wall and the metallic conductors is to be achieved. For example, when circuit board holes are drilled through a copper clad base plastic laminate or through a plastic laminate containing internal conductor planes such as in a multilayer circuit board, resin smear on the metallic surfaces exposed to the walls of the hole(s) must be removed to achieve proper functioning of the plated through holes.
Plated through-holes as described above are useful as electrical connection between printed circuits having metallic conductors on both sides or between two or more of the various planes and surface conductor layers in multilayer boards. The electrical and mechanical integrity required for this function can only be attained by insuring complete removal of resinous materials from the entire inner circumference of the portion of the metallic conducting layer exposed by the hole.
Another instance where resin smear removal is essential is in the formation of wire assemblies according to U.S. Pat. Nos. 3,646,572; 3,674,914; and 4,097,864, all to Burr, the disclosures of which are hereby incorporated by reference. This invention is known as a wire scribed circuit board and is manufactured under the tradename Multiwire.RTM.. Such wire assemblies may include, for example, networks of fine insulated wires affixed to a surface of a substrate. Interconnection between these wires and other conductors may be established by, for example, drilling a hole perpendicular to the wire planes of such a diameter and location that it intersects the wire, metallizing the barrel of the hole and the intersected tip of the wire, and soldering a connection to the metallized hole. If resin smears are not thoroughly removed from the hole wall and the wire tip, poor or no electrical contact between the metallized hole wall and the wire tip may result. Moreover, even if acceptable electrical contact should be initially achieved when an unclean hole is metallized, this contact may be lost in a later soldering operation due to expansion of the resin smear rupturing the physical contact between the metal hole wall and the intersected wire.
Numerous methods are known for removing resin smear. One approach is a mechanical one and involves channeling a dry or wet stream of abrasive particles through such holes. A similar method is the use of hydraulic pressure to force a thick slurry of abrasive material through the holes. Mechanical methods generally are relatively slow and difficult to control. Moreover, complete smear removal rarely is assured since good reproduceability with respect to all holes in a given circuit board is difficult to achieve.
Other methods of smear removal include the use of chemicals that attack the smeared resinous coating.
Concentrated sulfuric acid down to about 90 percent concentration has been successfully used. Smeared epoxy resin, which is usually less than about 0.001 inch thick, can be removed with about one minute's treatment with such an agent. Unfortunately, the high (92%-98%) sulfuric acid concentration required for effective smear removal also demands extraordinary precautions by operators. It produces undesirably rough hole walls. Concentrated sulfuric acid rapidly absorbs water and becomes ineffective, limiting its useful life span. Immersion time for smear removal varies with the absorbed water.
Another smear-removal agent is concentrated chromic acid. It is slower acting than sulfuric acid, requiring from five to fifteen minutes for equivalent smear removal. Operator caution and special tank linings and immersion racks are required. Increasingly strict water quality regulations and the difficulty of disposing of the concentrated chromic metal residuals in an ecologically satisfactory manner also weigh against this method of smear removal. In brief, chromic acid presents toxicity problems, difficulty of waste treatment of rinse water, and leaves chromium residues which interfere with electroless plating.
Various ratios and combinations of sulfuric and fluorine containing acids are used as smear-removal agents, but these are extremely toxic, requiring specially designed equipment.
Permanganate also has been used for smear removal for mechanical and laser drilled holes, as it does not require special equipment or extraordinary safety precautions for operators and presents little ecological hazard in waste disposal. See e.g., British Pat. No. 1,479,556 where the use of an alkaline permanganate solution having a pH of at least about 13 is disclosed. Permanganate solutions with a pH greater than 13 have been used for Multiwire.RTM. circuit boards to remove epoxy smear from the wire and etch back polyimide insulation around the wire. The use of high pH permanganate solution results in localized inactive areas and thus formation of pin holes or plating voids when subsequently electrolessly plated.
U.S. Pat. Nos. 4,042,729 and 4,054,693 disclose the use of permanganate solutions having a pH between 11 and 13 to achieve good adhesion of metal to a resinous substrate. This pH, however, is too low to accomplish rapid polyimide removal in wire scribed discrete wiring boards with polyimide coated wires.
In the process for making a wire scribed board, permanganate treatment has been used not only to remove the smear produced by a hole drilling step, but also to etch back the polyimide wire insulation, to some extent. Following the permanganate treatment, the hole is "neutralized" and rinsed. Neutralizers such as SnCl.sub.2, formaldehyde or hydrazine-hydrate solutions have been used. However, this permanganate treatment also reduces catalytic activity of the precatalyzed base material. As a result, it has been necessary to operate the electroless copper plating bath in two different modes.
In an initial mode in such prior art hole cleaning process, the copper plating bath produces sufficiently high activity to form a first copper layer. After formation of such first layer, the bath formulation is brought back to a less active and sufficiently stable condition to produce a copper deposit of adequate quality and thickness. However, the time to form the first copper layer is long. Switching between the two modes is sometimes difficult to control. And the copper plating bath can not be used on a continuous basis. This problem has been overcome by the improved hole cleaning solutions of the present invention.
Multilayer boards require a conventional seeded electroless process. This involves a separate catalysis step after hole cleaning. Another difficulty associated with multilayer processing is the formation of pin holes or plating voids in electroless plating after using very high pH permanganate hole cleaning solutions.
Hole cleaning for multilayer type boards and two-sided plated through boards has been accomplished by treating the boards with a permanganate solution, followed by neutralizing and rinsing. Since the treated boards typically are subsequently exposed to a seeding procedure, i.e. a palladium-tin-chloride solution, careful process control is, however, necessary to avoid the occurrence of "pinholes" or other plating voids. Otherwise, it is necessary to double seed or even to process twice both through the palladium-tin-chloride solution and electroless copper.
Leaving out the neutralizing step after permanganate treatment results in highly active species of permanganate residue on the substrate. Highly active permanganate residue in an electroless plating solution triggers formation of copper particles. Carrying this residue into the plating bath causes both instability and, after a relatively short use time, spontaneous decomposition of the bath.